Carrier sense multiple access method and wireless terminal apparatus

ABSTRACT

A carrier sense multiple access method capable of improving throughput in a wireless network is disclosed. This method is implemented in a wireless network—for example, a wireless LAN ( 100 )—having an AP ( 110 ) that is equipped with a multi-beam antenna ( 111 ) that forms a directional beam and that is capable of SDMA, and a STA ( 120, 130, 140 ) that communicate with the AP ( 110 ). With this method, the AP ( 110 ) allots the STA ( 120  to  140 ) to a beam group defined by a coverage area of the directional beam, according to the location of the STA ( 120  to  140 ). Then, the allocated STA ( 120  to  140 ) is assigned a signature signal for group identification, the signature signal being unique to the group.

TECHNICAL FIELD

The present invention relates to a carrier sense multiple access (CSMA)method, wireless base station apparatus and wireless terminal apparatus,used in a wireless network such as a wireless LAN (Local Area Network).

BACKGROUND ART

IEEE802.11 provides a cost-effective solution for networking terminalapparatuses including computers, for example, by wireless. With newdevelopments in signal processing and modulation technologies,enhancements have been made in standards supporting new physical layerswith higher data rates. Studies have shown that the key limiting factorin current 802.11 system is the MAC (Medium Access Control) layer, wherethroughput saturates with increase in data rates (see, for example,non-patent document 1). The IEEE802.11 working group has identified theneed for a high-throughput wireless LAN configured based on both MAC andPHY changes to existing wireless LANs.

For current applications and those envisioned for the future, the datarates supported by existing wireless LANs are sufficient. Wireless LANshave heretofore employed time division multiple access schemes, and theproblem with this lies in the number of users of high data rateapplications that the network can support at a time. The problem can begeneralized as one of the need for higher throughput. To achieve higherthroughput, it is necessary to improve data rates measured in layersabove layer 2, or the medium access control (MAC) layer, in the opensystem interconnection (OSI) by the International Organization forStandardization (ISO). To meet the requirement of increasing throughputfor all wireless terminal apparatuses in a typical wireless LAN systemcontaining a single wireless base station apparatus (for example, anaccess point (AP)) and a plurality of wireless terminal apparatuses(STAs), throughput is measured at the AP.

Communication in a conventional wireless LAN system is based on the CSMAscheme. That is, whether or not an STA is able to transmit a data packetis determined by detecting whether or not a medium to be accessed isbusy (occupied) or idle (unoccupied). One approach to improve throughputin this wireless LAN system is to exploit the benefits of the spacedivision multiple access (SDMA) scheme. However, optimum scheduling isnecessary for this purpose, such as, for example, performingtransmission and reception with different STAs with antennas ofdifferent APs.

Non-patent Document 1: “Throughput Analysis for IEEE 802.11a Higher DataRates”, doc.: IEEE 802.11/02-138r0, March 2002.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, in a conventional wireless LAN system, STAs access a medium bya contention-based access scheme (whereby a plurality of STAs contendfor the right to access the medium) such as the CSMA scheme, and holdtransmission of data packets when detecting that the medium is busy.Consequently, at each access timing, only one of a plurality of STAscontained in the wireless LAN system is able to perform data packettransmission. Consequently, it is not simple to improve throughput in awireless LAN system.

It is therefore an object of the present invention to provide a carriersense multiple access method, wireless base station apparatus andwireless terminal apparatus, that is capable of improving throughput ina wireless network.

Means for Solving the Problem

A carrier sense multiple access method according to the presentinvention is implemented in a wireless network having a wireless basestation apparatus that is capable of space division multiple access anda wireless terminal apparatus that communicates with the wireless basestation apparatus, the wireless base station apparatus having an antennasection that forms a directional beam and a plurality of transmissionand reception sections that are connected with the antenna section andare capable of parallel operation, and the method is configured with: anallocation step of allocating the wireless terminal apparatus to a groupdefined by a coverage area of the directional beam according to alocation of the wireless terminal apparatus; an assigning step ofassigning a signature signal to the wireless terminal apparatusallocated in the allocation step, the signature signal being unique tothe group; a detection step of detecting whether or not the signaturesignal assigned in the assigning step is present in a medium to beaccessed; and a transmission step of transmitting the signature signaland a data packet addressed to the wireless base station apparatusconcurrently when the signature signal is not present in the medium as aresult of the detection in the detection step.

A wireless base station apparatus according to the present invention iscapable of space division multiple access and has an antenna sectionthat forms a directional beam and a plurality of transmission andreception sections that are connected with the antenna section and arecapable of parallel operation, and the wireless base station apparatusemploys a configuration having: an allocation section that allocates awireless terminal apparatus to a group defined by a coverage area of thedirectional beam according to a location of the wireless terminalapparatus, the wireless terminal apparatus being a communicatingpartner; an assigning section that assigns a signature signal to theallocated wireless terminal apparatus, the signature signal being uniqueto the group; and a reporting section that reports the assignedsignature signal to the wireless terminal apparatus.

A wireless terminal apparatus according to the present invention employsa configuration having: a reception section that receives a signaturesignal, the signature signal being unique to a group defined by acoverage area of a directional beam formed by an antenna section of awireless base station apparatus and being reported from the wirelessbase station apparatus, the wireless base station apparatus being acommunicating partner; a detection section that detects whether or notthe signature signal is present in a medium to be accessed; a decrementsection that decrements a backoff value when the signature signal is notpresent in the medium as a result of the detection in the detectionsection; and a transmission section that transmits the signature signaland a data packet addressed to the wireless base station apparatusconcurrently when the backoff value is decremented down to zero by thedecrement section.

Advantageous Effect of the Invention

According to the present invention, it is possible to improve throughputin a wireless network.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a wireless LAN where a CSMA method according toan embodiment of the present invention is implemented;

FIG. 2 is a block diagram showing a configuration of a wireless LANaccording to an embodiment of the present invention;

FIG. 3 is a view for explaining access timing structure according to anembodiment of the present invention;

FIG. 4 is a view showing transmission pattern in uplink access period indetail according to an embodiment of the present invention;

FIG. 5 is a flowchart for explaining an enhanced CSMA algorithmaccording to an embodiment of the present invention;

FIG. 6 is a view for explaining a first example of a signature signaltransmission method according to an embodiment of the present invention;

FIG. 7 is a view for explaining a second example of a signature signaltransmission method according to an embodiment of the present invention;

FIG. 8 is a view for explaining a third example of a signature signaltransmission method according to an embodiment of the present invention;

FIG. 9 is a view for explaining an operation mode of an enhanced CSMAalgorithm according to an embodiment of the present invention; and

FIG. 10 is a timing chart of an enhanced CSMA algorithm according to anembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, embodiments of the present invention will be described below indetail with reference to the accompanying drawings.

A case will be described below with this embodiment where wirelessterminal apparatuses (hereinafter referred to as “STAs”) is allocated togroups determined according to the coverage range of directional beamsformed by the antenna section of a wireless base station apparatus (“AP”in this embodiment), and each group is assigned a unique signaturesignal so that each STA has a signature signal.

An AP and STAs will be described here that execute an CSMA algorithmaccording to an embodiment of the present invention (hereinafterreferred to as the “enhanced CSMA algorithm”) that enables a pluralityof STAs in a wireless LAN to concurrently perform medium contention anddata packet transmission and that therefore improves overall throughputof the wireless LAN. To help understand the present invention, thefollowing definitions will be used:

A “wireless LAN” refers to a wireless local area network, which mayinclude an arbitrary number of devices or nodes in order to provide LANservices to STAs through wireless access technologies.

A “station (STA)” refers to a device that is capable of accessingservices provided by a wireless LAN.

An “access point (AP)” refers to a wireless base station apparatus in awireless LAN that serves the role of controlling access to the wirelessLAN and maintaining access timing. The primary role of the AP is tofunction as a bridge so that STAs in a wireless LAN access devices inother networks.

“Space division multiple access (SDMA)” refers to an access mechanismwhereby a plurality of STAs can concurrently use the same band andperform communication, by virtue of their physical separation in spaceand the ability of the transmission and reception section to transmitand receive signals (data packets) using directional beams.

“Medium” refers to wireless channels where a wireless LAN operates.

A “multi-beam antenna” refers to an antenna system that is capable offorming directional beams in different directions with minimumcross-over and inter-beam interference so as to implement SDMA.

A “medium access control (MAC) layer” refers generally to the networkprotocol used by individual STAs in order to enable access to a medium.

A “physical (PHY) layer” refers to the actual transmission and receptionsection that transmits and receives signals in a wireless LAN, which canbe generalized to consist of several sub-layers including a convergencelayer from the MAC and a control layer.

“Uplink” refers to the direction of transmission from STA to AP.

“Downlink” refers to the direction of transmission from AP to STA (forexample, unicast, multicast or broadcast).

A “concurrent communication group” refers to a set of STAs that arecovered by one or more directional beams formed by an SDMA-capable APequipped with a multi-beam antenna.

A “beam range” refers to the coverage area of one or more directionalbeams formed concurrently by an AP.

A “beam group” refers to a set of STAs covered by one or moredirectional beams formed concurrently by an AP.

A “beam start beacon frame” refers to a frame broadcast by an AP overthe whole of a group's beam range so as to announce the start of thegroup's access period. It may be a frame which optionally containsinformation pertaining to the downlink period and schedule for thatbeam/group.

A “beam-end beacon frame” refers to a frame broadcast by an AP to STAsin a beam group/concurrent communication group so as to announce the endof the group's access period.

A “poll+supervised contention announcement frame” refers to a frame thatis broadcast by an AP to STAs belonging to beam groups so as to announceusers the start of an uplink access period, schedules for STAs toperform uplink access and the period during which access is allowed on acontention basis.

“Contention-based channel access” refers to a MAC mechanism wherebyindividual STAs access a shared medium based on a distributed contentionalgorithm executed by each STA without scheduling so as to ensurefairness in the medium access.

“Poll-based channel access” refers to a MAC mechanism whereby access toa shared medium is regulated by centralized control that allocatesaccess to the medium by polling individual STAs.

A “hidden terminal” refers to an STA that is located within an area thatcan be covered by beams irradiated from an AP yet is located outside anarea that can be covered by beams irradiated from another STA. Suchscenario may occur when a plurality of STAs are physically located atdiametrically opposite ends of a wireless LAN.

“Carrier sense multiple access (CSMA)” refers to a contention-basedchannel access technique that involves the process of detecting that amedium has been idle for a certain period of time, waiting for a randombackoff value and thereafter performing data packet transmission.

“Clear channel assessment (CCA)” refers to an algorithm that is used bya CSMA-based STA to determine the state of a medium (busy or idle).

A “signature signal” refers to a signal that is assigned to member STAsof each beam group, using which an STA determines whether or not otherSTAs in the same beam group active or transmitting.

A “slot time” is defined as a constant unit of time, including the CCAdetection time, round-trip propagation delay time and MAC processingtime.

A “detection-time” is defined as a constant unit of time required todetect whether or not a signature signal is present in a medium.

In the following descriptions, for purposes of explanation, specificnumbers, times, structures, protocol names and other parameters will begiven in order to provide a thorough understanding of the presentinvention. However, it is still obvious to one skilled in the art thatthe present invention may be implemented without these specific details.In other instances, well-known components and modules are shown in blockdiagrams in order not to obscure the present invention unnecessarily.

For a thorough understanding of the invention, in the followingdescription, some operation sequences, data structures and calculationtechniques for calculation will be presented. Certain data structureswill be used, serving only as an example of implementation of thepresent invention. It is obvious to one skilled in the art that in realimplementation, new information could be added, and certain parts couldbe omitted depending on the actual scenario that applies to the presentinvention.

FIG. 1 is a view showing a wireless LAN where a CSMA method according toan embodiment of the present invention is implemented.

Wireless LAN network 100 shown in FIG. 1 is a typical wireless LANconsisting of AP 110 and a plurality of STAs 120, 130 and 140. Inaddition, FIG. 2 provides a block diagram of the configuration ofwireless LAN 100.

STAs 120 to 140 basically have the same internal configuration, and soFIG. 2 shows only the internal configuration of STA 120 and omit theinternal configurations of STAs 130 and 140. Obviously, the STAsincluded in wireless LAN 100 are not limited to three STAs 120 to 140.In addition, in the following descriptions, one or more STAs included inwireless LAN 100 will be sometimes referred to as “user(s).”

AP 110 is a fixed network infrastructure device and has multi-beamantenna 111 that is capable of forming a plurality of spatiallyseparated directional beams 145 a, 145 b and 145 c and minimizinginter-beam crossover and interference. In addition, AP110 has: groupallocation section 112 that allocates STAs 120 to 140 in wireless LAN100 to beam groups according to the locations of STAs 120 to 140;signature signal assigning section that assigns signature signals, whichare unique to the respective groups, to allocated STAs 120 to 140; and aplurality of transmission and reception sections 114-1, 114-2 . . .114-N that are connected with multi-beam antenna 111 and are capable ofparallel operation and that govern wireless communication of STAs 120 to140 including signaling of signature signals to STAs 120 to 140, andtransmission and reception of data packets with STAs 120 to 140. Below,an unspecified one of transmission and reception sections 114-1 to 114-Nwill be referred to as “transmission and reception section 114.” Inaddition, like enhanced CSMA algorithm execution section 123 in STAs 120to 140, AP 110 has enhanced CSMA algorithm execution section 115 forexecuting an enhanced CSMA algorithm.

Generally, STA 120 is small due to the requirements of portability andmobility, and uses a simple antenna (usually consisting of only oneelement), which is capable of forming an omni-directional or nearlyomni-directional beam radiation pattern. In addition, STA 120 has:transmission and reception section 122 that governs wirelesscommunication with AP 110, including reception of signature signalssignaled from AP 110 and transmission and reception of data packets withAP 110; and enhanced CSMA algorithm execution section 123 that executesan enhanced CSMA algorithm when transmission is going to be made to AP110. The enhanced CSMA algorithm will be described later in detail. STAs130 and 140 have the same configuration as that of STA120 and implementthe same operations.

AP 110 is equipped with multi-beam antenna 111 and is capable of forminga set of beams directed in a plurality of different directions includingan omni-directional beam radiation pattern.

Depending on the spatial or angular separation between users (forexample, STAs 120 to 140), their traffic patterns and the capabilitiesof the access point including the number of beams that can be formed andtheir resolution, group assigning section 112 assign users to beamgroups, so as to optimize the use of medium 150. Then, signature signalassigning section 113 assigns, to allocated users, signature signalsthat are uniquely associated with the respective groups.

Signature signal assignment is performed in the group allocation phase(that is, while users are assigned to concurrent communication groupsand beam groups). In addition, signature signals may be reassigned atany time users move or are reallocated to new concurrent communicationgroups or beam groups.

By thus assigning a signature signal to data packets when a usertransmits data packets, it is possible to determine whether other userscontending for medium 150 are users of the same beam group. If a userperforms transmission while another user from the same beam group isperforming transmission, collision occurs and transmission is held. Itis also possible to determine whether users of different beam groups areperforming transmission. In this case, AP 110 is able to receive twotransmissions separately based on angular separation between a pluralityof users, thereby enabling a plurality of users contending for medium150 to perform transmission at the same time.

Then, transmission and reception section 114 reports signature signalsto users through one-to-one frame exchange or information elementsignaling exchange. AP 110 signals the start and end of the period usersin beam groups are allowed to access medium 150 to users, therebyproviding services to users in beam groups in time division.

Traffic in wireless LAN 100 is characterized as uplink or downlink. Toprevent collisions in other beams (based on the assumption that STAs useomni-directional antennas) and collisions produced by AP 110 itself (dueto imperfect isolation in practical design of RF components), thetransmission and reception operations must always be kept synchronizedat AP 110 for all beams. To be more specific, synchronization ismaintained for periods that are assigned to a plurality of users ofconcurrent communication groups and divided for uplink or downlink. Inaddition, the occurrence of these periods is synchronized betweendifferent beam groups in a concurrent communication group. This timingstructure will be described with reference to FIG. 3.

FIG. 3 is a view showing an example of an access timing structure whereAP 110 divides users in wireless LAN 100 in two concurrent communicationgroups X and Y. An explanation will be given here with primary focus onaccess period for concurrent communication group X.

Users belonging to concurrent communication group X are allowed accessin access period 151. Users belonging to concurrent communication groupY are allowed access in access period 152. Concurrent communicationgroups X and Y are comprised of three concurrently formed beam groups A,B and C. Transmission pattern 153 is for users belonging to beam groupA. Transmission pattern 154 is for users belonging to beam group B.Transmission pattern 155 is for users belonging to beam group C.

Access period 151 for concurrent communication group 151 is divided indownlink access period 156 and uplink access period 157. Referencenumeral 158 marks time position alignment and synchronization ofdownlink access period 156 and uplink access period 157 in a concurrentcommunication group.

Beam start beacon frame 159 is broadcast by AP 110. Beam start beaconframe 159 marks the start of access period 151 assigned to concurrentcommunication group X, the length of downlink access period 156, and,optionally, downlink transmission schedules in beam groups A, B and C.

Poll+supervised contention announcement frame 160 is the last frametransmitted in downlink access period 156 and announces schedules for aplurality of users under differing beam groups A, B and C of concurrentcommunication group X. Poll+supervised contention announcement frame 160also announces the remaining period access is possible on a contentionbasis after the scheduled transmissions of a plurality of users underbeam groups A, B and C are completed. Reference numeral 161 designates abeam end beacon frame marking an end of access period 151 for aplurality users belonging to concurrent communication group X. Beam endbeacon frame 161 is broadcast by AP 110. Similar transmission patternsapply to all other concurrent communication groups including concurrentcommunication group Y.

Network traffic may be classified according to statistics such as datarates, packet inter-arrival rates and packet delay bounds. Generic MACmechanisms include the contention-based mechanism and the poll-basedmechanism. Poll-based channel access is an efficient mechanism fortraffic that requires a certain QoS, which may be characterized in termsof certain band and delay bounds. Examples of such traffic includeaudio/video, voice and other multimedia contents. On the other hand,contention-based channel access is a mechanism intended to serve trafficthat is random and non-periodic, including hyper-text transferprotocols, etc.

As mentioned above, SDMA is subject to a stringent synchronizationrequirement. Consequently, poll-based channel access is favorable, inthat, by optimally scheduling uplink and downlink traffic of differentbeam groups, AP 110 can efficiently spatially reuse the channel band,resulting in higher overall throughput. It is obvious that trafficemanating from AP 110, that is, downlink traffic, can be easilyscheduled because AP 110 knows traffic characteristics. However, withregularly recurring uplink traffic, the user (transmission source) isgenerally required to make a resource reservation so that AP 110 is ableto grant band in poll+supervised contention announcement frame 160.

FIG. 4 illustrates transmission patterns in uplink access period 157described with reference to FIG. 3 in detail.

An example of the transmission pattern of beam group A of concurrentcommunication group X will be described here, where transmission ofpoll+supervised contention announcement frame 160, which marks the endof downlink access period 156, announces schedules for beam groups A, Band C in uplink access period 157.

During uplink access period 157, users first perform transmission inpoll-based access periods 171, 172 and 173 in accordance with theschedules announced in poll+contention announcement frame 160. Afterscheduled poll-based access periods 171 to 173 are over, the remainingperiods are used as contention-based access periods 174, 175 and 176.During poll-based channel access, users determine timing to performtransmission based on their announced schedules and local clocks.

A carrier sense technology underlies the mechanism of contention-basedchannel access, which includes the process of confirming that a mediumhas been idle for a certain period of time before attempting attransmission. In SDMA-based wireless LAN 100 where AP 110 employsmulti-beam antenna 111 while users employ simpler and smallerform-factor omni-directional antenna 121, a user belonging to beam groupA—that is, a user located in the beam range of beam group A—would holdor defer transmission while a user belonging to beam group B or beamgroup C is performing transmission. Multi-beam antenna 111 hasdirectivity, so that AP 110 is able to resolve a plurality oftransmissions originating from different beam ranges. Situation mayoccur where, due to physical separation between a plurality of users anduse of power control alone, users located in different beam rangesbecome “hidden terminals” and as a result end up accessing uplinkconcurrently. However, in reality, users are rarely spread in such away.

Based on the example of FIG. 4, poll-based access period 173 of beamgroup C ends earlier than poll-based access periods 171 and 172 of beamgroups A and B. As a result, contention-based access period 176 of beamgroup C starts earlier than contention-based access periods 174 and 175of beam groups A and B.

AP 110 is able to resolve a plurality of transmissions originating fromseparate areas. A user belonging to beam group C is able to startcontention-based channel access without waiting for the end oftransmission in poll-based access periods 171 and 172, by utilizing theCSMA algorithm according to the present invention. Likewise, as for beamgroups A and B, once poll-based access periods 171 and 172 of the groupsare over, contention-based access periods 174 and 175 can be started.Consequently, waste of channel band can be reduced largely. In addition,users belonging to beam groups A, B and C are able to access a mediumconcurrently, so that, at each timing, the number of users that can beallowed access can be increased compared to prior art.

Next, the enhanced CSMA algorithm executed in STA 120 (also in STAs 130and 140) will be described. FIG. 5 is a flowchart for explaining anenhanced CSMA algorithm according to an embodiment of the presentinvention.

First, in step S701, the backoff counter value is rest to 0. Followingthis, in step S702, a CCA algorithm is used to determine whether medium150 is busy or idle. The CCA algorithm is based on received power levelof the channel. According to the IEEE 802.11a specification, there aretwo requirements on detection probability depending on whether or notthe preamble of a signal is detected. STA 120 generally usesomni-directional antenna 121, so that, when there is an STA performingtransmission in a given beam range, medium 150 is determined to be busyat an STA in another beam range.

As a result of the CCA algorithm, if medium 150 is determined to be busy(S702: YES), the flow proceeds following the first stage S703 side, andfirst proceeds to step S704. In step S704, whether or not a data packetthat is going to be transmitted is new—that is, whether or not thetransmission is the first transmission attempt—is determined. When thedata packet is new (S704: YES), the flow proceeds to step S705, and,when the data packet is not new—that is, when the transmission is anattempt for the second time or more (S704: NO)—the flow proceeds to stepS751. Incidentally, first stage S703 represents a conventional CSMAalgorithm.

In step S705, whether the idle duration time of medium 150 has reached(or become the same as or exceeded) DIFS (Distributed Inter-Frame Space)is determined. When the idle duration time is the same as or hasexceeded DIFS (S705: YES), the flow proceeds to step S715 and a datapacket is transmitted immediately. On the other hand, when the idleduration time is less than DIFS (S705: NO), the flow proceeds to stepS751.

In step S751, whether or not the idle duration time of the medium hasreached DIFS is determined. When medium 150 changes to a busy statebefore the idle duration time reaches DIFS (S751: NO), the flowimmediately returns to step S702, and, when the idle state continuesuntil reaching DIFS (S751: YES), the flow proceeds to step S752.

In step S752, as in step S704, whether or not a data packet that isgoing to be transmitted is new—that is, whether or not the transmissionis the first transmission attempt—is determined. If this is the firsttransmission attempt (S752: YES), a random backoff value is selected andset as the backoff counter value (S706), and then the flow proceeds tostep S753. In the event of a transmission attempt for the second time ormore (S752: NO), no action is incurred and the flow simply proceeds tostep S753. The random backoff value is a value set as a random integermultiple of a slot time.

In step S753, whether or not the backoff counter value is 0 isdetermined. When the backoff counter value is 0 (S753: YES), the flowproceeds to step S715 and a data packet is immediately transmitted. Whenthe backoff counter value is not 0 (S753: NO), the flow proceeds to stepS714.

In step S714, whether or not the idle state of medium 150 continues forthe slot time is determined. As a result of this determination, if thestate changes to a busy state before reaching the slot time (S714: NO),the flow returns to step S702. When the idle state continues for theslot time (S714: YES), the backoff counter value is decremented by onein step S754. After the decrement, the flow returns again to step S753and steps S753, S714 and S754 repeat until the backoff counter valuebecomes 0 and transmission is performed or until medium 150 enters abusy state.

Meanwhile, in step S702, if medium 150 is determined to be in a busystate (S702: NO), the flow proceeds to second stage S708. Second stageS708 represents part of the enhanced CSMA algorithm. In step S709, whichis the first step in second stage S708, like in step S751, whether ornot a data packet that is going to be transmitted is new—that is,whether the transmission is the first transmission attempt—isdetermined. If the transmission is the first transmission attempt (S709:YES), a random backoff value is selected and set as the backoff countervalue (S710) and then the flow proceeds to step S711. If thetransmission is a transmission attempt for the second time o more, noaction is incurred and the flow simply proceeds to step S711.

In step S711, whether or not a signature signal of the same group asthat of the STA attempting at transmission of this data packet isdetected, is determined. IF the signature signal of the same beam groupis detected—that is, if another STA in the same beam group istransmitting a data packet (S711: YES)—the flow returns to step S702,and, if the signature signal is not detected (S711: NO), the flowproceeds to step S755.

In step S755, whether or not the backoff counter value is 0 isdetermined. If the backoff counter value is 0 (S755: YES), the datapacket is immediately transmitted (S715). If the backoff counter valueis not 0, the flow proceeds to next step S756.

In step S756, whether medium 150 is in an idle state or busy state isdetermined using CCA. If medium 150 is determined to be idle (S756:YES), the flow immediately proceeds to step S714 of first stage S703. Ifmedium 150 is still in a busy state (S756: NO), the flow proceeds tostep S757.

In step S757, again, whether or not the signature signal of the samebeam group is detected is determined. The determination period in thisstep is the detection time. In this step, if the signature signal of thesame beam group is detected (S757: YES), the flow immediately returns tostep S702. On the other hand, if the signature signal of the same beamgroup is not detected (S757: NO), the flow proceeds to step S758 and thebackoff counter value is decremented by one. After the decrement, theflow returns to step S755 again.

While the decrement (S754) in the process of first stage S703 is made inslot time units, the decrement (S758) in the process of second stageS708 is made in detection time units, because, depending on the natureof the signature signal used, the detection time can be longer than theslot time. In this case, if the backoff counter value is decremented inslot time units, the backoff counter value may become 0 before thedetection time of one signature signal is over. This suggests that, as aresult, transmission is carried out only to make collision. Decrementingin detection time units therefore may prevent the occurrence of suchsituations.

A primary requirement in the above-noted enhanced CSMA algorithm is thatSTA 120 is able to detect the signature signals of other STAs 130 and140 in medium 150 where a plurality of transmissions are performedconcurrently.

The method of transmitting signature signals will be described below inthree examples. FIG. 6 is a view for explaining the first example of thesignature signal transmission method. FIG. 7 is a view for explainingthe second example of the signature signal transmission method. FIG. 8is a view for explaining the third example of the signature signaltransmission method.

Wireless LAN 100, operating in the 5 GHz UNII (Unlicensed NationalInformation Infrastructure) band, operates using 20 MHz channelization.In the example shown in FIG. 6, the frequency of the zero-th subcarrier182 (that is, the center part of the spectrum used), which conforms to20 MHz spectral mask 181 in OFDM (Orthogonal Frequency DivisionMultiplexing) transmission and which nevertheless is not used in OFDMtransmission 183 usually carrying data packets alone.

In other words, in OFDM transmission 184 for data packets and signaturesignal, the signature signal is transmitted in low power using thezero-th subcarrier 185.

Although the channelization in the 5 GHz UNII band is 20 MHz, thebaseband spectral occupancy is 16.667 MHz, as shown by reference numeral186. In other words, the remaining band—that is, the 20 MHZ spectralmask minus 16.667 MHz spectral occupancy 186—is used as a guard band,provided so as to combat roll-off produced in filter design.

In the example shown in FIG. 7, the frequency of the ±27th subcarriers191 that are located at the boundaries of the spectrum used and are notused in usual OFDM transmission 183 carrying data packets alone, isused.

That is, in OFDM transmission 192 of data packets and signature signal,the signature signal is transmitted using the ±27th subcarriers 193. Thetransmission of the ±27th subcarriers 193 is performed in lower powerthan transmission of other subcarriers, so as to moderate therequirement imposed on the filter of the receiving end by reducing powerof additional subcarriers.

In the example shown in FIG. 7, twice as much spectrum are obtained forthe transmission of signature signals compared to the example of FIG. 6,and consequently it is possible to transmit twice as much signaturesignals. By this means, it is possible to double the code space or thebeam range that can be addressed by a given length of code and reducethe detection time by a factor of two. Also, it is possible tofacilitate the synchronization of the correlator and code at thereceiving end.

Incidentally, the zero-th subcarrier 185 and ±27th subcarriers 193 maybe used concurrently.

In all the above-described examples, the frequency band that is not usedfor data packet transmission can be utilized for signature signaltransmission.

A plurality of users belonging to differing beam groups may transmitsignature signals concurrently, so that each user is able to determinewhether or not the signature signal the same as the one assigned theretoby AP 110 is being transmitted. Consequently, preferably, for example, aspreading code may be used as a signature signal. By using spreadingcodes having high autocorrelation characteristics, signature signaldetection can be performed highly successfully. In addition, by usingspreading codes having low cross-correlation characteristics, theprobability of error detection in signature signals can be reduced. Bythus using spreading codes as signature signals, the presence or absenceof signature signals suggesting whether or not other users in the samebeam group have already been performing transmission can be determinedaccurately using a correlator (matched filter).

In addition, as shown in FIG. 6, when the zero-th subcarrier 185 is usedfor transmission of signature signals, signature signals may be givencharacteristics such that their average value becomes zero. By thismeans, it is possible to minimize the impact of DC offset which occursin the transmitting user's D/A converter and in the receiving user's A/Dconverter.

In addition, if a spreading code is used as a signature signal, even iftransmission of a signature signal is performed in lower transmissionpower than transmission of a data packet, the low power can becompensated for by the processing gain at the receiving end.

Instead of using a spreading code as a signature signal, it is alsopossible to utilize the orthogonality between subcarriers in thefrequency domain. As shown in FIG. 8, three orthogonal subcarriers194—that is, the ±27th and 0th subcarriers—are obtained. Thesesubcarriers 194 can be used as separately identified signature signals.In this example, three signature signals are associated respectivelywith the ±27th and zero-th subcarriers. Although according thismechanism the number of signature signals may be limited to the numberof subcarriers, signature signal detection may be performed in highspeed by measuring the power of individual frequencies by the apparatusof the receiving end.

To support the concept of signature signals, the enhanced CSMA algorithmneeds to be modified to support two state-determining variables, namelythe CCA (Clear Channel Assessment) value and the signature signaldetection value, as shown in FIG. 6. These values determine the threeoperation modes shown in FIG. 9 as decrement time units, namelyoperation modes 604, 605 and 606 marked by the slot time, detectiontime, and N/A (no transmission) time.

An example of operation where there are three STAs (referred to as “STA1,” “STA 2” and “STA 3” in this example) will be described following theoperation mode shown in FIG. 9 and the flowchart of the enhanced CSMAalgorithm of FIG. 5. STA 1 and STA 2 belong to the same beam group andshare the same signature signal, while the third, STA3, belongs to adifferent beam group and at the same time has a different signaturesignal. Initially, assume that medium 150 is busy, STA 1 selects arandom backoff value of 6 and STA 2 selects a random backoff value of 4.As shown in FIG. 10, STA3 starts transmission at time instant 801. Upuntil this instant, STA1 and STA2 decrement their respective backoffcounter values, as shown in their slot periods (that is, periods 802 and803). Once STA 3 starts transmission, the CCA algorithm detects thatmedium 150 is busy and, as a result, the backoff counter value isdecremented one by one. Periods 804 and 805 indicate that the backoffcounter values of STA 1 and STA 2 are reduced in detection time units.

At time 806, the random backoff of STA 2 (selected to be 4) is exhaustedand STA 2 starts transmission. By this means, STA 1 needs to switch itsoperation mode to the operation mode designated by the reference numeral606 during period 807 in which STA 2 performs transmission, and holdtransmission. When the transmission by STA 2 is completed, STA 1 is nolonger able to detect that medium 150 is busy using the CCA algorithm,and so switches to the operation mode designated by the referencenumeral 604 in FIG. 9. After the backoff counter value is decrementeddown to zero (in slot time units), STA 1 acquires the right to accessmedium 150 and starts transmission at the instant designated by thereference numeral 808.

According to this embodiment, STAs 120 to 140 are allocated to beamgroups defined by the coverage areas of directional beams formed bymulti-beam antenna 111 of AP 110 according to the locations of STA 120to 140, and signature signals, which are unique to the respectivegroups, are assigned to allocated STAs 120 to 140, so that, when aplurality of STAs that contend for medium 150 at the same time belong todifferent beam groups, AP 110 has a transmission and reception sectionon a per beam basis and the STAs can communicate concurrently withSDMA-capable AP 110, and throughout in wireless LAN 100 can be improved.

Incidentally, wireless LAN 100 described in this embodiment has severalapplicable technical fields. That is, in companies/corporations, usersmay use wireless access-capable notebook computers at their desks andmove these to meeting rooms and elsewhere. At home, an AP may beconnected to a home AV server which may consist of a set top box, mediaplayer, and portal to the Internet, and devices equipped with wirelessaccess functions such as display panels, cameras and notebook computersmay be used to access the Internet or medium stored in the home AVserver.

In addition, wireless LAN 100 according to this embodiment is applicableto cellular hotspots such as office building lobbies or coffee shopswhere STAs using data services may gain access.

The present specification is based on Japanese Patent Application No.2003-324793, filed on Sep. 17, 2003, the entire content of which isexpressly incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The carrier sense multiple access method, wireless base stationapparatus and wireless terminal apparatus of the present invention havethe advantage of improving throughput in a wireless network and areapplicable in a wireless network such as a wireless LAN.

1. A carrier sense multiple access method implemented in a wirelessnetwork comprising a plurality of wireless terminal apparatuses and awireless base station apparatus, said wireless base station apparatusbeing capable of space division multiple access and comprising anantenna section that forms a directional beam according to locations ofthe plurality of wireless terminal apparatuses and a plurality oftransmission and reception sections that are connected with said antennasection and are capable of parallel operation, the method comprising: anallocation step of allocating, in the wireless base station apparatus,the plurality of wireless terminal apparatuses to groups defined bycoverage areas of the directional beam; an assigning step of assigning,in the wireless base station apparatus, signature signals to theplurality of wireless terminal apparatuses allocated in the allocationstep, each of the signature signals being unique to each of the groups;a detection step of detecting, in at least one of the plurality ofwireless terminal apparatuses, whether or not a signature signalassigned in the assigning step is present in a medium to be accessed; adetermination step of determining, in the at least one wireless terminalapparatus, whether or not the medium is being used for data packettransmission; a decrement step of: decrementing, in the at least onewireless terminal apparatus, a backoff value at every slot time when themedium is determined to be idle; decrementing, in the at least onewireless terminal apparatus, the backoff value at every detection timewhen the medium is determined to be being used for data packettransmission by another wireless terminal apparatus allocated to adifferent group from that of the at least one wireless terminalapparatus; and not decrementing, in the at least one wireless terminalapparatus, the backoff value until an end of data packet transmissionusing the medium by another wireless terminal apparatus allocated to thesame group as that of the at least one wireless terminal apparatus, whenthe medium is determined to be being used for data packet transmissionby the other wireless terminal apparatus allocated to the same group;and a transmission step of concurrently transmitting, from the at leastone wireless terminal apparatus, the signature signal and a data packetaddressed to the wireless base station apparatus when the signaturesignal allocated to the same group is not present in the medium as aresult of the detection in the detection step and the backoff value isdecremented to zero in the decrement step, wherein: every detection timeis longer than the slot time.
 2. The carrier sense multiple accessmethod according to claim 1, wherein, in the transmission step, in aband used in communication, a frequency of a center part of said bandthat is not used for transmission of the data packet is used fortransmission of the signature signal.
 3. The carrier sense multipleaccess method according to claim 2, where the signature signaltransmitted using the frequency of the center part has a characteristicsuch that an average value of said signature signal is zero.
 4. Thecarrier sense multiple access method according to claim 1, wherein, inthe transmission step, in a band used in communication, frequencies ofboundaries of said band that are not used for transmission of the datapacket are used for transmission of the signature signal.
 5. The carriersense multiple access method according, to claim 1, wherein, in thetransmission step, in a band used in communication, frequencies of acenter part and boundaries of said band that are not used fortransmission of the data packet are used for transmission of thesignature signal.
 6. The carrier sense multiple access method accordingto claim 1, wherein: the signature signal is associated on a per groupbasis with a plurality of frequencies in a band used in communicationthat are not used for transmission of the data packet in thetransmission step; and in the transmission step, the signature signal istransmitted using the frequencies associated with said signature signal.7. The carrier sense multiple access method according to claim 1,wherein: the signature signal has an autocorrelation characteristicabove a predetermined level and a cross correlation characteristic belowsaid predetermined level; and in the detection step, the presence orabsence of the signature signal is detected by performing a correlationcalculation using said signature signal.
 8. The carrier sense multipleaccess method according to claim 1, wherein, in the transmission step,transmission of the signature signal is performed in lower transmissionpower than in transmission of the data packet.
 9. A wireless terminalapparatus comprising: a reception section that receives a signaturesignal transmitted from a wireless base station apparatus that is acommunication partner, the signature signal being unique to each ofgroups defined by coverage areas of a directional beam formed by anantenna section of the wireless base station apparatus according tolocations of a plurality of wireless terminal apparatuses including thewireless terminal apparatus; a detection section that detects whether ornot the signature signal is present in a medium to be accessed; adetermination section that determines whether or not the medium is beingused for data packet transmission; a decrement section that: decrementsa backoff value at every slot time when the medium is determined to beidle; decrements the backoff value at every detection time when themedium is determined to be being used for data packet transmission byanother wireless terminal apparatus allocated to a different group fromthat of the wireless terminal apparatus; and not decrementing thebackoff value until an end of data packet transmission using the mediumby another wireless terminal apparatus allocated to the same group asthat of the wireless terminal apparatus, when the medium is determinedto be being used for data packet transmission by the other wirelessterminal apparatus allocated to the same group; and a transmissionsection that transmits concurrently the signature signal and a datapacket addressed to the wireless base station apparatus when thesignature signal allocated to the same group is not present in themedium as a result of the detection in the detection section and thebackoff value is decremented to zero in the decrement section, wherein:every detection time is longer than the slot time.